Metallurgy of tin



No Drawing.

superficial film of o zinc slowly dissolves,

Patented June 24, 1930 UNITED STATES PATENT OFFICE- WALTER 0. SNELLING, OF ALLENTOWN,

METALLURGY OF TIN Application filed March 5,

My invention relates to the recovery of tin from its oxide compounds, and more particularly my invention relates to a process of recovering tin from cassiterite, its principal oxide ore. The principal object of my present invention is to provide an efficient and inexpensive method for the 'preparation of metallic tin or chemical salts of tin directly from the mineral cassiterite, which forms the principal commercially available ore of tin.

In U. S. Patent 917,176 of April 6, 1909 there is described a process of treating cassiterite by disposing the mineral, in suitably sub-divided form or in naturally occuring forms such as stream tin, wood tin, etc. in contact with an electrode in an electrolyte capable of liberating hydrogen. Upon passage of an electric current reduction of the ore occurs, with the production of a metallic tin upon the grams or particles of cassiterite. The tin thus reduced is thereafter transferred by electrical action to a suitable cathode, from which it may be removed from time to time.

In practice it has been found that the above described process, although operative, requires the expenditure of a large amount of electrical energy per unit of tin produced, and one of the objects of my present inven' tion is to provide means for the more econoniical production of tin' from its oxide ores by suitable modifications of the described process.

When particles of cassiterite are in contact with metallic zinc in the presence of a suitable electrolyte, such as dilute sulfuric acid or dilute hydrochloric acid, for ex ample, an electrolytic couple results. The

and a chemically equivalent amount of hydrogen is evolved upon the surface of the particles of cassiterite, with superficial reduction of the tin oxide to metallic tin. By carefully examining a particle of cassiterite in contact with a piece of zinc in an electrolyte of dilute hydrochloric acid, for example, the formation of a thin layer of metallic tin can be observed to occur, gradually spreading from the point of contact of the zinc with the particle of cassiterite,

particle of PENNSYLVANIA until the entire surface of the cassiterite will finally showa metallic appearance indicating the complete reduction of the surface of the mineral to metallic tin, while chemical examination of the hydrochloric acid electrolyte will show the presence of zinc the solution of the although apparently ily proved to be elec ing particles of cass ZIIIC.

chloride resulting from The effect noted, chemical, may be readtrolytic, and by dispositerite upon a plate of any suitable metal in an electrolyte comprising dilute hydrochloric acid, for example, and passing an electric current from any suitable anode to the ode, an exactly simil face film of metallic occur.

Although the su cassiterite to metall about with plate of metal as cathar production'of a surtin may be observed to perficial' reduction of ic tin can be brought comparative ease by direct electrical reduction, with the cassiterite forming the cathode of an electrolytic cell, or

may be brought about indirectly, by disposing cassiterite in contact with an electropositive metal such as zinc, for example, in

any suitable electrolyte, and

particularly in an electrolyte comprising a dilute solution of hydrochloric acid or sulfuric acid, and although the superficial film of metallic tin so formed may trically and forth in U.

be readily transferred elecdeposited, in S. 917,176,

unexpected and hitherto unexplained difficulties have followed efforts to rope at this cycle time after time. In the preliminary electrolytic reduction of the cassiterite, good electrical efficiency is obtained,

stripping of the reduced film and in the electrolytic of metallic tin,

correspondingly good electrical efficiency is obtained, but for some reason, until now un own, upon repe second time, the ove of the cycle is found ating this same cycle a rall electrical efficiency to be lower than in the preliminary cycle, and upon each repetition first 10% of the inn may be obtained in cathode by the expen -0f the cycle the electrical efiiciency shows a still further reduction,

so that although the present in the cassiterite mossy form upon the diture of only a moderate amount of energy, the obtaining of the next 10% of the tin present in the cassiterite requires an entirely disproportionate amount of energy, and by the time that half of the cassiterite has been reduced and transferred, the electrical requirements of the process have become so great, and the electrical wastage in the cycle has become so large, that the process 1S economically unsuitable to compete with direct smelting methods.

Since it is hardly reasonable to believe that a cycle which can be initially conducted with high efficiency will show lower efliciency upon being repeated, and will show constantly decreased efficiency when repeated a number of times, except in the presence of some inhibiting factor, 1 have investigated the electrolytic reduction of cassiterite for the purpose of determining the cause of the behaviour noted, and I have now discovered the reason for this behaviour and means for its elimination.

Although metallic tin is anexcellent conductor of electricity cassiterite is a very poor conductor comparing in this respect with silicon dioxide, its chemical analog. Accordingly, it is necessary that a particle of cassiterite should be in direct contact with a cathodic electrode, in order to undergo partial reduction, but upon preliminary reduction occurring, the film of metallic tin that is formed serves to soon coat the particle of cassiterite with a superficial coating of reduced tin, and I have found that this operation involves only slightly more than the theoretical requirement of electrical energy, more energy being evolved when hydrogen unites with the oxygen of the cassiterite than is. evolved when hydrogen is evolved upon the surface of the reduced tin as the free gas. Accordingly, in the case of any particle of cassiterite, the reduction of the tin oxide to metallic tin will proceed with relatively small requirement of energy, as long as any of the cassiterite uncoated by tin is present in electrical contact with either the cathodic electrode or reduced tin on either a partially coated or completely coated grain or lump of cassiterite. This explains the relatively low requirement of electrical energy in the first cycle of reduction, metallic tin forming readily at the point of contact with the cathodic electrode, and extending as a surface film progressively over the entire surface of the grain of cassiterite, and thereafter over any grains or particles of cassiterite contacting with the film of reduced tin upon the grain or lump first reduced. Careful tests have indicated that practically no free hydrogen is produced when a mass of cassiterite is first treated in the manner described, and that the reduction of the cassiterite takes place progressively over the surface until all of the cassiterite present is superficially reduced.

7 desirable contact with memes Upon now removing this superficial coating of tin, either chemically or electrically, and repeating the cycle a second time, some free hydrogen will be evolved, and the reduction of the cassiterite particles will be observed to go on at a much slower rate than before, and upon each repetition of the cycle a constantly increasing amount of free hydrogen is observed to be liberated, with corresponding reduction in the efficiency of the current in the reduction of the tin. Careful examination of the particles of cassiterite, after ten cycles and after twenty cycles, for example, has afforded a positive explanation of the phenomenon, in the form of a surface layer of non-conducting oxides of tantalum, silicon and iron. These oxides represent impurities originally present in the cassiterite, and they are not reduced and transferred with the tin, and as a result they accumulate upon the surface of the cassiterite particles, constantly reducing the ability to obtain good contact with the cathodic electrode, until finally after repeating the cycle a large number of times the thickness of the insulating layer upon the cassiterite particles becomes so great as to practically prevent further reduction'of the tin oxide still present in the interior of the grain of partially reduced cassiterite.

'1 have discovered that by reducing cassiterite to form a superficial film of metallic tin, stripping this reduced tineither electrolytically or chemically by the action of strong hot hydrochloric acid, for example, and then mechanically removing the film of insulating impurities before electrolytic reduction in the next succeeding cycle, I can wholly avoid the difiiculties hitherto met with, and can completely reduce cassiterite to metallic tin of high purity.

In my preferred method of procedure I first superficially reduce cassiterite electrolytically, according to well known methods, by either cathodic reduction or contact with an electro-positive metal such as zinc in the presence of an electrolyte such as hydrochloric acid or sulfuric acid. I may next, strip the superficially reduced tin electrolytically, or I may strip it by the action of hot hydrochloric acid, where tin chloride is desired for chemical uses, or as an intermediate product for the subsequent electrolytic winning of the tin. The cassiterite, after superficial reduction and stripping of the tin, now has over its surface a film representing the impurities present in the original cassiterite, and this film is in eneral of extreme thinness, although its lnsulating characteristics are such that even when present in' the smallest amount, it prevents the cathodic electrode. I have found that upon removing this film, by such mechanical means for exelectrolytically ample asrotating the cliarge of cassiteriteina ball millifor a fewsseconds,.or byxshalrzing oragitatihg a basket in; which thecas siterite :i's-.held;, or by brushing or abradingor by'any" equivalent means, I can restore theonce-treated cassiterite to. its original: condition, so that upon subsequent electro lytic reduction: it will function the same as: fresh cassitenite, and may be: reduced with-- out. the) expenditure of. an; excessive amount of electrical energy.

'It will; oi course be evident that instead. of removing; the superficial film of insulating oxides present upon the surface of the cassiterite after one or more; cycles of treat-- ment as described, by mechanical brushe ingorrubbing action, or-by anyother: form of attrition), the oxides may he removed by chemical". means. By the of hydrofluoric acid, for example, is an excellent solvent of silicon dioxide, I can remove portions of the film and materially increase the efficiency of the electrolytic cycle, although the mechanical removal of the film is so 25, simple that in general I prefer to employ mechanical attrition alone, in the removal \of the interferingfilm represented by the impurities originally present in the cassiterite.

I do not claim as any part of my present invention the electrolitic reduction of cassiterite to metallic tin, as this has long been known, and neither do I claim as any part of my present invention the electrolytic transfer of tin from superficially reduced cassiterite, as a means of obtaining the metal in mossy form. My present invention relates specifically to the process which comprises removing a surface film of insulating impurities from cassiterite, as a step in the electrolytic winning of metallic tin. Although I prefer to remove this superficial lic tin, stripping the reduced tin from the surface of the cassiterite and mechanically removing insulating impurities from the resulting surface of the cassiterite.

In testimony whereof, I have hereunto subscribed my name this 3rd day of March,

' WALTER 0. SN ELLINGL insulating film intermittently, following a stripping operation for the removal of metallic tin and preceding the operation of again forming a superficial coating of metalllctin-upon the surface of the eassiterite, it will of course be evident that the l operation of removing the superficial layer ofinsulating materials may be conducted concurrently with either the reducing operation or the stripping operation, and accordingly I wish to broadly claim the step of removing substantially tin-free insulating substances from the surface of cassiterite in the electrolytic winning of tin therefrom, regardless of the stage in the electrolytic treatment in which it is removed.

I claim: 1. As a step in the electrolytic winning of tin from cassiterite, the process which comprises removing insulating bodies from the surface of the electrolytically-reduced cassiterite.

2. As a step in the electrolytic winning of 

